Tiers are used to allow the PO (Pokémon Online) community to divide Pokémon into groups based on how good they are. Basically, tiers allow us to create competitive environments where weaker Pokémon can be used without being completely overpowered by stronger ones.

Brief Main Tier SummaryOU (Over Used) is the primary and most popular tier that allows most Pokémon, but strong Pokémon such as Arceus and Mewtwo are banned. The tiers below OU: UU (Under Used), RU (Rarely Used), NU (Never Used), and PU restrict more Pokémon as the tier level decreases, which creates opportunities for Pokémon with less usage to shine.

Ubers is the highest tier where almost every Pokémon is allowed. LC (Little Cup) is the lowest tier, and it only allows Pokémon that have not evolved, and battles are set at level 5. Note that Ubers and LC are not usage tiers, which means that Pokémon cannot be added or removed from these tiers based on usage.

Tier Listing
A listing of the Pokémon allowed in each tier can be found here.